“Never doubt that a small group of thoughtful,
committed people can change the world. Indeed, it is the
only thing that ever has. “
I started this biped project in 2006. I felt like I was
climbing the highest mountain in the world on my own.
This year, Chris and Girts joined up and we became a team.
Now, I can see the top. This month’s article is a team
effort. Each member is working on different parts of the
biped, so it is divided into the three sections: mechanical
changes, the biped systems, and the Gazebo simulation.

Mechanical Changes —
Daniel

Finding servos that are high torque that don’t require
you to take a second mortgage on your house is not an
easy task. Our original servos claimed over 400 oz/in of
torque, but they just could not deliver those numbers. We
have just placed an order with a company in China that has
a giant size R/C servo for under $100 with a claimed 850
oz/in of torque.

In the meantime, we have made some significant
changes in the biped. Using the Gazebo simulator, we
finally came up with some hard torque numbers. Strong
servos are crucial for two joints in particular. The hip in/out
and the knee servos need about 1,600 and 1,200,
respectively, to lift and hold the upper body weight.

The first change we made was to replace one of our
original servos that could not hold the hip in place with a
super strong albeit heavy 3,200 oz/in Torxis servo from
Invenscience. Unlike hobby servos, I was not able to back-drive these. This works out nice when the biped is standing
still or when that joint doesn’t need to move.

These are powerful servos built with strong output
shafts. I can simply shut them down and they hold their
positions. At under $300 each, I would have used them for

by Daniel Albert, Chris Mayer and Girts Linde

38 SERVO 11.2013

The Road
to the DARPA
Robotics
Challenge

Go to www.servomagazine.com/index.php?/magazine/
article/november2013_Albert to comment on this article.